What are the causes of long-term fuel trim being too high?
2 Answers
The reasons for long-term fuel trim being too high are: 1. Clogged fuel injectors and poor spark plug combustion; 2. Poor quality gasoline; 3. Measurement deviation of the air flow meter and air leakage; 4. Long-term failure to replace the fuel filter, resulting in low fuel pressure; 5. Oxygen sensor detecting excessively high oxygen content. The suitable range for long-term fuel trim is between -23% and 16%. The following factors affect the fuel trim coefficient: 1. Engine misfire: When the engine misfires, incomplete combustion occurs in the cylinder, leading to high levels of HC and O2 in the exhaust. Since HC does not easily participate in catalytic reactions, oxygen consumption is low, and residual oxygen levels are high, causing the oxygen sensor to measure a lower voltage, indicating a lean mixture. The ECU then increases fuel injection, making the short-term fuel trim correction value positive. 2. Poor exhaust flow: A clogged exhaust pipe obstructs the discharge of exhaust gases, manifesting as excessive backpressure and reduced intake of fresh air into the cylinders. This results in incomplete combustion of the mixture, with the oxygen sensor voltage exceeding 0.45V, and the fuel trim coefficient becoming negative. 3. Low fuel pump pressure or clogged fuel injectors: Clogged fuel injectors cause symptoms such as sluggish acceleration and lack of power at high speeds. The engine remains in a lean mixture state.
Speaking of long-term fuel trim being too high, I've encountered it several times myself. Simply put, the engine control unit thinks the air-fuel mixture is too lean, so it has to inject more fuel to balance it. The most common cause is an air leak, such as a broken vacuum hose or poor intake manifold sealing, which allows air to enter and dilute the mixture. Another issue is a faulty O2 sensor—incorrect readings can trick the computer into adding too much fuel. My car once had skyrocketing fuel consumption because of this, and it was frustrating to drive. Eventually, I found out the fuel injectors were clogged with carbon deposits, and after cleaning them, things returned to normal. Additionally, an exhaust leak can mislead the oxygen readings, and insufficient fuel pressure—due to a failing pump or clogged filter—can lead to inadequate fuel injection, requiring higher trim adjustments. In short, don’t ignore it—get it checked promptly, or your car will lose power, consume more fuel, and even risk engine damage.
